With the exception of potassium, magnesium is the most abundant intracellular ion. It is essential to many physiochemical processes including the activation of ATP in the transfer of energy rich phosphate, the activation of enzymes involved in lipid, carbohydrate, and protein metabolism, and the preservation of the macromolecular structure of DNA, RNA and ribosomes. Magnesium also has a significant influence on the neuromuscular apparatus and decreased concentrations of magnesium may result in tetany and convulsions, while increased levels can cause general anesthesia, respiratory failure and cardiac arrest. Because tetany due to reduced magnesium concentrations is clinically indistinguishable from that caused by low calcium levels it is frequently necessary to perform assays for both serum magnesium and calcium at the same time.
Many methods have been used to determine magnesium levels, including phosphate precipitation techniques, complexometric titration procedures, fluorescent spectrophotometry, and dye absorption methods utilizing Titan yellow. These methods are generally time consuming or suffer from technical drawbacks. The best method for the assay of magnesium is generally considered to be atomic absorption spectrophotometry, however this requires expensive instrumentation which often makes it impracticable for smaller laboratories. The determination of magnesium by reaction with calmagite is also known however with this methodology the reagents are somewhat unstable and the determination in general subject to a variety of interferents.
In general, the existing methodologies for measuring total calcium in biologic fluids involves considerable manipulation of samples and reagents prior to determination. Gravimetric and titrimetric methods usually require large sample volumes. Colorimetric methods, both manual and automated, commonly involve final readings under highly alkaline conditions. The indicators used for such determinations are often unstable at the final pH thus requiring reagent dilution with a strong base. The measurement of calcium and magnesium with chlorophosphonazo III is known, however, the concurrent measurement of both calcium and magnesium with the chlorophosphonazo III methodology requires radical pH changes in order to measure one ion without interference by the other, as well as close control of experimental conditions for reliable results.